Techniques for increasing efficiency of a waveguide of a lidar system

ABSTRACT

A light detection and ranging apparatus (LIDAR) includes at least one waveguide including a first cladding layer having a first refractive index and a second cladding layer having multiple second refractive indexes to expand an optical mode of an optical beam propagating within the waveguide. The second refractive indexes include a gradient of refractive indexes and the first refractive index is less than the second refractive indexes.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/545,070 filed Dec. 8, 2021, now U.S. Pat. No. 11,408,980, to beissued Aug. 9, 2022, which claims priority and the benefit of U.S.patent application Ser. No. 17/188,072 filed Mar. 1, 2021, now U.S. Pat.No. 11,237,250, issued Feb. 1, 2022, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to light detection and ranging(LIDAR) that provides simultaneous measurement of range and velocityacross two dimensions.

BACKGROUND

Fast-scanning mirrors are the primary components used to illuminate ascene in most conventional LIDAR systems. One mirror typically scansquickly along the X direction (azimuth), while another mirror scansslowly along the Y direction (elevation). Light emission and detectionfrom target reflections are done coaxially, typically via a single-modefiber. A 3D point cloud can be established when point-wise detectedrange information is combined with angular position feedback from thescanning mirrors.

To achieve higher frame rates, the mirror's angular velocity isincreased, especially that of the scanner in faster scan direction, suchas along the X direction. When using mirrors with a high angularvelocity and single-mode fiber-based detection, a target signal fromdistant objects can be severely degraded. Signal degradation is oftendue to the difference in angular position of the scanner mirror from thelaunch time of the optical signal (pulsed or frequency swept) to thecollection time of the same signal from a distant scattering target.This slight angular change causes a walk-off of the target signal at thefiber tip decreasing the coupling efficiency, which manifests itself asa weaker signal detection. Such degradation becomes more severe as thefiber diameter decreases or as the mirrors' angular velocity increases.

SUMMARY

Example implementations of the present disclosure are directed to animproved scanning LIDAR system. Example implementations of the presentdisclosure are based on a type of LIDAR that uses frequency modulation(FM) and coherent detection to overcome the shortcomings of traditionalLIDAR systems and the limitations of prior FM LIDAR systems.Historically, FM LIDAR systems suffer from significant losses in thebeam's return path; thus, such systems, which are often quite bulky,require a higher average beam output power to measure distancescomparable to time-of-flight (TOF) LIDAR systems. However, the range islimited by the operating distance for eye-safe output powers.

Example implementations of the present disclosure are configured tosimultaneously measure range and velocity, using coherent detection, andhave the added benefit of immunity to crosstalk from other LIDARsystems. Other implementations may be used with incoherent systems toimprove, range, frame rate, or detection. Example implementationsminimize optical losses in the beam's return path, thereby increasingthe system's measurement range. Additionally, by using nondegenerateoptical sources, example implementations can leverage mature wavelengthdivision multiplexing (WDM) techniques often used in integrated siliconphotonics, a desired platform due to its compactness and relativestability in varying environmental conditions.

A conventional frequency-modulated continuous-wave (FMCW) LIDAR systemrelies on scanning an optical beam, such as a laser beam, or multipleoptical beams over a desired field of view (FOV) to map target space inthree dimensions (3D) and in time. To guide the optical beam, aconventional LIDAR system may utilize one or more waveguides. Eachconventional waveguide may include a first cladding layer (also referredto as a “p-cladding layer” hereafter) and a second cladding layer (alsoreferred to as an “n-cladding layer” hereafter) that have relativelysimilar thicknesses and constant refractive indexes. While aconventional waveguide may be suitable for guiding the optical beam, thedesign of the conventional waveguide may be improved to increase theperformance of the conventional FMCW LIDAR system.

A conventional quantum well or multi-quantum well (MQW) may use anoptical source to generate light. The MQW and a separate confinementheterostructure (SCH) may form a waveguide, where P-doped and N-dopedlayers may form the cladding layers of the waveguide. The SCH may beformed of a material having a step or graded refractive index. Thewaveguide, however, does not include an n-cladding layer that has arange of different refractive indexes.

Example implementations of the present disclosure provide for animproved, graded refractive index waveguide (also referred to as“waveguide” hereafter). The waveguide may include a first cladding layer(e.g., p-cladding layer) and a second cladding layer (e.g., n-claddinglayer), as previously described. The second cladding layer of thewaveguide, however, may be composed of a material having a range ofrefractive indexes that are higher than the first cladding layer. Forexample, the second cladding layer may have a refractive index gradientthat increases from a first surface of the second cladding layer to asecond surface of the second cladding layer. Furthermore, the thicknessof the second cladding layer may be greater than the thickness of thefirst cladding layer.

Use of the waveguide may provide for an improved LIDAR system. The rangeof refractive indexes of the n-cladding layer may pull and expand theoptical mode of the optical beam into the n-cladding layer from thep-cladding layer. This may result in an increase in the saturation powerof the optical beam because the optical confinement factor anddifferential gain are significantly reduced. This may also result inimproved coupling efficiency by expanding the beam profile vertically tomatch another waveguide of the LIDAR system, improved efficiency of theoptical beam, and reduce amplitude and phase noises experienced by theoptical beam, all of which improve the performance of the LIDAR system.

The present disclosure includes, without limitation, the followingexample implementations.

Some example implementations provide a light detection and ranging(LIDAR) system including an optical source to generate an optical beamand one or more waveguides, coupled to the optical source, to steer theoptical beam. The one or more waveguides include a first cladding layerhaving a first refractive index and a second cladding layer disposedbelow the first cladding layer and above a substrate. The secondcladding layer includes second refractive indexes that steer the opticalbeam towards the substrate. The second refractive indexes include arange of different refractive indexes, wherein the range of differentrefractive indexes is greater than the first refractive index of thefirst cladding layer.

Some example implementations provide a method including receiving, by awaveguide, an optical beam from an optical source of a light detectionand ranging (LIDAR) system. The waveguide includes a first claddinglayer having a first refractive index and a second cladding layerdisposed below the first cladding layer. The second cladding layerincludes second refractive indexes that steer the optical beam towards asubstrate. The second refractive indexes include a range of differentrefractive indexes, wherein the range of different refractive indexes isgreater than the first refractive index of the first cladding layer. Thewaveguide propagates the optical beam using the range of differentrefractive indexes of the waveguide to expand an optical mode of theoptical beam.

Some example implementations provide a LIDAR system including an opticalsource to generate an optical beam and one or more waveguides coupled tothe optical source to steer the optical beam. The one or more waveguidesinclude a first cladding layer having a first refractive index, a secondcladding layer disposed below the first cladding layer, the secondcladding layer having a second refractive index, wherein the secondrefractive index is greater than the first refractive index, and a thirdcladding layer disposed below the second cladding layer, the thirdcladding layer having a third refractive index, wherein the thirdrefractive index is greater than the second refractive index and thefirst refractive index.

These and other features, aspects, and advantages of the presentdisclosure will be apparent from a reading of the following detaileddescription together with the accompanying figures, which are brieflydescribed below. The present disclosure includes any combination of two,three, four or more features or elements set forth in this disclosure,regardless of whether such features or elements are expressly combinedor otherwise recited in a specific example implementation describedherein. This disclosure is intended to be read holistically such thatany separable features or elements of the disclosure, in any of itsaspects and example implementations, should be viewed as combinableunless the context of the disclosure clearly dictates otherwise.

It will therefore be appreciated that this Brief Summary is providedmerely for purposes of summarizing some example implementations so as toprovide a basic understanding of some aspects of the disclosure.Accordingly, it will be appreciated that the above described exampleimplementations are merely examples and should not be construed tonarrow the scope or spirit of the disclosure in any way. Other exampleimplementations, aspects, and advantages will become apparent from thefollowing detailed description taken in conjunction with theaccompanying figures which illustrate, by way of example, the principlesof some described example implementations.

BRIEF DESCRIPTION OF THE FIGURE(S)

Embodiments and implementations of the present disclosure will beunderstood more fully from the detailed description given below and fromthe accompanying drawings of various aspects and implementations of thedisclosure, which, however, should not be taken to limit the disclosureto the specific embodiments or implementations, but are for explanationand understanding only.

FIG. 1 illustrates a LIDAR system according to example implementationsof the present disclosure.

FIG. 2 is a time-frequency diagram of an FMCW scanning signal that canbe used by a LIDAR system to scan a target environment according to someembodiments.

FIG. 3 depicts a waveguide having a graded index cladding layer, inaccordance with embodiments of the disclosure.

FIG. 4 is an illustration of an example of a cross-section of awaveguide transmitting an optical beam, in accordance with embodimentsof the disclosure.

FIG. 5 is an illustration of an example of a waveguide having multiplecladding layers with different refractive indexes, in accordance withembodiments of the disclosure.

FIG. 6 depicts a flow diagram of a method for utilizing a graded indexwaveguide by a LIDAR system in accordance with implementations of thepresent disclosure.

DETAILED DESCRIPTION

According to some embodiments, the described LIDAR system describedherein may be implemented in any sensing market, such as, but notlimited to, transportation, manufacturing, metrology, medical, virtualreality, augmented reality, and security systems. According to someembodiments, the described LIDAR system is implemented as part of afront-end of frequency modulated continuous-wave (FMCW) device thatassists with spatial awareness for automated driver assist systems, orself-driving vehicles.

FIG. 1 illustrates a LIDAR system 100 according to exampleimplementations of the present disclosure. The LIDAR system 100 includesone or more of each of a number of components, but may include fewer oradditional components than shown in FIG. 1. One or more of thecomponents depicted in FIG. 1 can be implemented on a photonics chip,according to some embodiments. The optical circuits 101 may include acombination of active optical components and passive optical components.Active optical components may generate, amplify, and/or detect opticalsignals and the like. In some examples, the active optical componentincludes optical beams at different wavelengths, and includes one ormore optical amplifiers, one or more optical detectors, or the like.

Free space optics 115 may include one or more optical waveguides tocarry optical signals, and route and manipulate optical signals toappropriate input/output ports of the active optical circuit. Inembodiments, the one or more optical waveguides may include one or moregraded index waveguides, as will be described in additional detail belowat FIGS. 3-6. The free space optics 115 may also include one or moreoptical components such as taps, wavelength division multiplexers (WDM),splitters/combiners, polarization beam splitters (PBS), collimators,couplers or the like. In some examples, the free space optics 115 mayinclude components to transform the polarization state and directreceived polarized light to optical detectors using a PBS, for example.The free space optics 115 may further include a diffractive element todeflect optical beams having different frequencies at different anglesalong an axis (e.g., a fast-axis).

In some examples, the LIDAR system 100 includes an optical scanner 102that includes one or more scanning mirrors that are rotatable along anaxis (e.g., a slow-axis) that is orthogonal or substantially orthogonalto the fast-axis of the diffractive element to steer optical signals toscan an environment according to a scanning pattern. For instance, thescanning mirrors may be rotatable by one or more galvanometers. Objectsin the target environment may scatter an incident light into a returnoptical beam or a target return signal. The optical scanner 102 alsocollects the return optical beam or the target return signal, which maybe returned to the passive optical circuit component of the opticalcircuits 101. For example, the return optical beam may be directed to anoptical detector by a polarization beam splitter. In addition to themirrors and galvanometers, the optical scanner 102 may includecomponents such as a quarter-wave plate, lens, anti-reflective coatedwindow or the like.

To control and support the optical circuits 101 and optical scanner 102,the LIDAR system 100 includes LIDAR control systems 110. The LIDARcontrol systems 110 may include a processing device for the LIDAR system100. In some examples, the processing device may be one or moregeneral-purpose processing devices such as a microprocessor, centralprocessing unit, or the like. More particularly, the processing devicemay be complex instruction set computing (CISC) microprocessor, reducedinstruction set computer (RISC) microprocessor, very long instructionword (VLIW) microprocessor, or processor implementing other instructionsets, or processors implementing a combination of instruction sets. Theprocessing device may also be one or more special-purpose processingdevices such as an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), a digital signal processor (DSP),network processor, or the like.

In some examples, the LIDAR control systems 110 may include a signalprocessing unit 112 such as a DSP. The LIDAR control systems 110 areconfigured to output digital control signals to control optical drivers103. In some examples, the digital control signals may be converted toanalog signals through signal conversion unit 106. For example, thesignal conversion unit 106 may include a digital-to-analog converter.The optical drivers 103 may then provide drive signals to active opticalcomponents of optical circuits 101 to drive optical sources such aslasers and amplifiers. In some examples, several optical drivers 103 andsignal conversion units 106 may be provided to drive multiple opticalsources.

The LIDAR control systems 110 are also configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the galvanometers of the optical scanner 102 based oncontrol signals received from the LIDAR control systems 110. Forexample, a digital-to-analog converter may convert coordinate routinginformation from the LIDAR control systems 110 to signals interpretableby the galvanometers in the optical scanner 102. In some examples, amotion control system 105 may also return information to the LIDARcontrol systems 110 about the position or operation of components of theoptical scanner 102. For example, an analog-to-digital converter may inturn convert information about the galvanometers' position to a signalinterpretable by the LIDAR control systems 110.

The LIDAR control systems 110 are further configured to analyze incomingdigital signals. In this regard, the LIDAR system 100 includes opticalreceivers 104 to measure one or more beams received by optical circuits101. For example, a reference beam receiver may measure the amplitude ofa reference beam from the active optical component, and ananalog-to-digital converter converts signals from the reference receiverto signals interpretable by the LIDAR control systems 110. Targetreceivers measure the optical signal that carries information about therange and velocity of a target in the form of a beat frequency,modulated optical signal. The reflected beam may be mixed with a secondsignal from a local oscillator. The optical receivers 104 may include ahigh-speed analog-to-digital converter to convert signals from thetarget receiver to signals interpretable by the LIDAR control systems110. In some examples, the signals from the optical receivers 104 may besubject to signal conditioning by signal conditioning unit 107 prior toreceipt by the LIDAR control systems 110. For example, the signals fromthe optical receivers 104 may be provided to an operational amplifierfor amplification of the received signals and the amplified signals maybe provided to the LIDAR control systems 110.

In some applications, the LIDAR system 100 may additionally include oneor more imaging devices 108 configured to capture images of theenvironment, a global positioning system 109 configured to provide ageographic location of the system, or other sensor inputs. The LIDARsystem 100 may also include an image processing system 114. The imageprocessing system 114 can be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LIDAR control systems 110 or other systemsconnected to the LIDAR system 100.

In operation according to some examples, the LIDAR system 100 isconfigured to use nondegenerate optical sources to simultaneouslymeasure range and velocity across two dimensions. This capability allowsfor real-time, long range measurements of range, velocity, azimuth, andelevation of the surrounding environment.

In some examples, the scanning process begins with the optical drivers103 and LIDAR control systems 110. The LIDAR control systems 110instruct, e.g., via signal processor unit 112, the optical drivers 103to independently modulate one or more optical beams, and these modulatedsignals propagate through the optical circuits 101 to the free spaceoptics 115. The free space optics 115 directs the light at the opticalscanner 102 that scans a target environment over a preprogrammed patterndefined by the motion control system 105. The optical circuits 101 mayalso include a polarization wave plate (PWP) to transform thepolarization of the light as it leaves the optical circuits 101. In someexamples, the polarization wave plate may be a quarter-wave plate or ahalf-wave plate. A portion of the polarized light may also be reflectedback to the optical circuits 101. For example, lensing or collimatingsystems used in LIDAR system 100 may have natural reflective propertiesor a reflective coating to reflect a portion of the light back to theoptical circuits 101.

Optical signals reflected back from an environment pass through theoptical circuits 101 to the optical receivers 104. Because thepolarization of the light has been transformed, it may be reflected by apolarization beam splitter along with the portion of polarized lightthat was reflected back to the optical circuits 101. In such scenarios,rather than returning to the same fiber or waveguide serving as anoptical source, the reflected signals can be reflected to separateoptical receivers 104. These signals interfere with one another andgenerate a combined signal. The combined signal can then be reflected tothe optical receivers 104. Also, each beam signal that returns from thetarget environment may produces a time-shifted waveform. The temporalphase difference between the two waveforms generates a beat frequencymeasured on the optical receivers 104 (e.g., photodetectors).

The analog signals from the optical receivers 104 are converted todigital signals by the signal conditioning unit 107. These digitalsignals are then sent to the LIDAR control systems 110. A signalprocessing unit 112 may then receive the digital signals to furtherprocess and interpret them. In some embodiments, the signal processingunit 112 also receives position data from the motion control system 105and galvanometers (not shown) as well as image data from the imageprocessing system 114. The signal processing unit 112 can then generate3D point cloud data that includes information about range and/orvelocity points in the target environment as the optical scanner 102scans additional points. The signal processing unit 112 can also overlay3D point cloud data with image data to determine velocity and/ordistance of objects in the surrounding area. The signal processing unit112 also processes the satellite-based navigation location data toprovide data related to a specific global location.

FIG. 2 is a time-frequency diagram 200 of an FMCW scanning signal 201that can be used by a LIDAR system, such as system 100, to scan a targetenvironment according to some embodiments. In one example, the scanningwaveform 201, labeled as f_(FM)(t), is a sawtooth waveform (sawtooth“chirp”) with a chirp bandwidth Δfc and a chirp period Tc. The slope ofthe sawtooth is given as k=(Δfc/Tc). FIG. 2 also depicts target returnsignal 202 according to some embodiments. Target return signal 202,labeled as f_(FM)(t-Δt), is a time-delayed version of the scanningsignal 201, where Δt is the round trip time to and from a targetilluminated by scanning signal 201. The round trip time is given asΔt=2R/ν, where R is the target range and ν is the velocity of theoptical beam, which is the speed of light c. The target range, R, cantherefore be calculated as R=c(Δt/2). When the return signal 202 isoptically mixed with the scanning signal, a range dependent differencefrequency (“beat frequency”) Δf_(R)(t) is generated. The beat frequencyΔf_(R)(t) is linearly related to the time delay Δt by the slope of thesawtooth k. That is, Δf_(R)(t)=kΔt. Since the target range R isproportional to Δt, the target range R can be calculated asR=(c/2)(Δf_(R)(t)/k). That is, the range R is linearly related to thebeat frequency Δf_(R)(t). The beat frequency Δf_(R)(t) can be generated,for example, as an analog signal in optical receivers 104 of system 100.The beat frequency can then be digitized by an analog-to-digitalconverter (ADC), for example, in a signal conditioning unit such assignal conditioning unit 107 in LIDAR system 100. The digitized beatfrequency signal can then be digitally processed, for example, in asignal processing unit, such as signal processing unit 112 in system100. It should be noted that the target return signal 202 will, ingeneral, also includes a frequency offset (Doppler shift) if the targethas a velocity relative to the LIDAR system 100. The Doppler shift canbe determined separately, and used to correct the frequency of thereturn signal, so the Doppler shift is not shown in FIG. 2 forsimplicity and ease of explanation. It should also be noted that thesampling frequency of the ADC will determine the highest beat frequencythat can be processed by the system without aliasing. In general, thehighest frequency that can be processed is one-half of the samplingfrequency (i.e., the “Nyquist limit”). In one example, and withoutlimitation, if the sampling frequency of the ADC is 1 gigahertz, thenthe highest beat frequency that can be processed without aliasing(Δf_(Rmax)) iS 500 megahertz. This limit in turn determines the maximumrange of the system as R_(max)=(c/2)(Δf_(Rmax)/k) which can be adjustedby changing the chirp slope k. In one example, while the data samplesfrom the ADC may be continuous, the subsequent digital processingdescribed below may be partitioned into “time segments” that can beassociated with some periodicity in the LIDAR system 100. In oneexample, and without limitation, a time segment might correspond to apredetermined number of chirp periods T, or a number of full rotationsin azimuth by the optical scanner.

FIG. 3 depicts a waveguide 300 having a graded index cladding layer, inaccordance with embodiments of the disclosure. In some embodiments,waveguide 300 may be implemented in a LIDAR system, such as the LIDARsystem 100 of FIG. 1. The waveguide 300 may include a first claddinglayer 302 (e.g., p-cladding layer) and a second cladding layer 304(e.g., n-cladding layer). In FIG. 3, the waveguide 300 is illustrated asbeing grown to a substrate 306. In some embodiments, the waveguide 300may be a ridge-waveguide (RWG) that includes a conductive ridgeprotruding into the waveguide that has a width (e.g., RWG width 316).

In some embodiments, the manufacturing of waveguide 300 may includegrowing the layers (e.g. first cladding layer 302, second cladding layer304, etc.) of waveguide 300 on a substrate 306. For example, themanufacturing process may begin with a raw substrate 306 and the layersof waveguide 300 may be grown sequentially, layer by layer, using ametal organic chemical vapor deposition (MOCVD) or molecular-beamepitaxy (MBE) reactor.

In embodiments, the first cladding layer 302 may be formed of a materialthat has a constant refractive index. A refractive index of a materialmay correspond to a numerical value that describes how quickly anoptical signal travels through the material. In some embodiments, thefirst cladding layer 302 may be formed of a material that has a gradedrefractive index. Example materials that may be used to form the firstcladding layer 302 may include, but are not limited to indium phosphide(InP) or indium gallium arsenide phosphide (InGaAsP).

The second cladding layer 304 may be disposed below the first claddinglayer 302 and formed of a material that has a range of refractiveindexes. In some embodiments, the second cladding layer 304 may be indirect contact with the first cladding layer 302. In other embodiments,one or more intervening layers, such as an active region and separateconfinement heterostructure (SCH), may be disposed between the secondcladding layer 304 and the first cladding layer 302.

In embodiments, the range of refractive indexes of the second claddinglayer 304 may correspond to a graded index or gradient of refractiveindexes that increases from the first surface 312 of the second claddinglayer 304 to the second surface 314 of the second cladding layer 304. Inan embodiment, the range of refractive indexes of the second claddinglayer 304 may correspond to a graded index or gradient of refractiveindexes that decreases from the first surface 312 of the second claddinglayer 304 to the second surface 314 of the second cladding layer 304. Insome embodiments, the range of refractive indexes may change at a linearrate. In an embodiment, the range of refractive indexes may change at anexponential rate. In some embodiments, the range of refractive indexesmay change according to any other rate. Example materials that may beused to form the second cladding layer 304 may include, but are notlimited to indium phosphide (InP), indium gallium arsenide phosphide(InGaAsP), indium gallium aluminum arsenide (InGaAlAs), or anycombination thereof.

In waveguide 300, the range of refractive indexes of the second claddinglayer 304 may be greater than the refractive index of the first claddinglayer 302. For example, if the first cladding layer 302 has a refractiveindex value of 3.15, the second cladding layer 304 may have a range ofrefractive index values from 3.2-3.3. The range of refractive indexes ofthe second cladding layer 304 being greater than the refractive index ofthe first cladding layer 302 may cause the optical mode of an opticalbeam being transmitted through waveguide 300 to be expanded (e.g.,pulled) towards the substrate 306, as will be described in additionaldetail at FIG. 4 below.

The first cladding layer 302 may have a thickness 308 and the secondcladding layer 304 may have a thickness 310 that each correspond to thedimensional height of their respective cladding layers that formwaveguide 300. In embodiments, the thickness 308 of the first claddinglayer 302 may be less than the thickness 310 of the second claddinglayer 304. In an embodiment, the thickness of the first cladding layermay be between 1.2 and 1.8 microns (μm). In some embodiments, thethickness 310 of the second cladding layer 304 may be between 2 and 5μm.

It should be noted that thickness 308 of the first cladding layer 302and thickness 310 of second cladding layer 304 are shown forillustrative purposes only and may not be to scale in accordance withembodiments of the disclosure.

FIG. 4 is an illustration of an example of a waveguide 400 expanding theoptical mode of an optical beam, in accordance with embodiments of thedisclosure. Waveguide 400 may correspond to waveguide 300, as previouslydescribed in FIG. 3, which is transmitting an optical beam 402. In FIG.4, waveguide may 400 be illustrated in an X-Y plane and the optical beam402 may be transmitted through waveguide 400 along a Z-axis (not shown)that is perpendicular to the X-Y plane.

Referring to FIG. 4, the range of refractive indexes of the secondcladding layer 304 being greater than the refractive index of the firstcladding layer 302 pulls the optical beam 402 towards the substrate 306,resulting in an expanded optical mode 404 of the optical beam 402. Insome embodiments, the expanded optical mode 404 of the optical beam 402may reduce the effect of the refractive index difference introduced by aRWG. Therefore, a wider RWG (e.g., RWG width 316) may be required tohave a single guided spatial mode. Having a wider RWG may provide forincreased saturation power, reduced thermal and electrical impedances,reduced current spreading, and reduced far-field divergence angle, whichcan improve the performance of the waveguide and associated LIDARsystems described herein.

Furthermore, the expanded optical mode 404 of the optical beam 402 mayreduce the optical internal losses as the optical beam 402 istransmitted via the waveguide 400, which results in more efficientoptical signals, higher wall plug efficiency (WPE), and reducesamplitude and phase noises typically experienced. In some embodiments,because the optical mode of the optical beam 402 has a relatively smalloverlap with the first cladding layer 302 dopants may be added to thefirst cladding layer 302 to further reduce electrical and thermalimpedances, improving the efficiency of the waveguide 400 and associatedLiDAR systems described herein.

FIG. 5 is an illustration of an example of a waveguide 500 havingmultiple cladding layers with different refractive indexes, inaccordance with embodiments of the disclosure. In embodiments, waveguide500 may be implemented in a LIDAR system, such as the LIDAR system 100of FIG. 1. The waveguide 500 may include a first cladding layer 502, asecond cladding layer 504, and a third cladding layer 506. In FIG. 5,the waveguide 500 is illustrated as being grown on a substrate 508, aspreviously described at FIG. 3.

In embodiments, the first cladding layer 502 may be formed of a materialthat has a first refractive index. Example materials that may be used toform the first cladding layer 502 may include, but are not limited toindium phosphide (InP) or indium gallium arsenide phosphide (InGaAsP).

The second cladding layer 504 may be disposed below the first claddinglayer 502 and may be formed of a material that has a second refractiveindex. In embodiments, the second refractive index of the secondcladding layer 504 may be greater than the first refractive index of thefirst cladding layer 502. Example materials that may be used to form thesecond cladding layer 504 may include, but are not limited to indiumphosphide (InP), indium gallium arsenide phosphide (InGaAsP), indiumgallium aluminum arsenide (InGaAlAs), or any combination thereof.

The third cladding layer 506 may be disposed below the second claddinglayer 504 and may be formed of a material that has a third refractiveindex. In embodiments, third refractive index of the third claddinglayer 506 may be greater than the second refractive index of the secondcladding layer 504 and the first refractive index of the first claddinglayer 502. Example materials that may be used to form the third claddinglayer 506 may include, but are not limited to, indium phosphide (InP),indium gallium arsenide phosphide (InGaAsP), indium gallium aluminumarsenide (InGaAlAs), or any combination thereof.

The third refractive index of the third cladding layer 506 being greaterthan the second refractive index of the second cladding layer 504 andthe first refractive index of the first cladding layer 502 may cause theoptical mode of an optical beam being transmitted by waveguide 300 to beexpanded (e.g., pulled) towards the substrate 508, as previouslydescribed at FIG. 4. In some embodiments, the second cladding layer 504and/or the third cladding layer 506 may be formed of a material thatincludes a range of refractive indexes, as previously described at FIG.3.

In some embodiments, the second cladding layer 504 may be in directcontact with the first cladding layer 502 and/or the third claddinglayer 506. In other embodiments, one or more intervening layers may bedisposed between the second cladding layer 504 and the first claddinglayer 502 and/or the second cladding layer 504 and the third claddinglayer 506.

The first cladding layer 502 may have a thickness 510, the secondcladding layer 504 may have a thickness 512, and the third claddinglayer 506 may have a thickness 514 that each correspond to thedimensional height of their respective cladding layers that formwaveguide 500. In some embodiments, the thickness 510 of the firstcladding layer 502 may be less than a combination of the thickness 512of the second cladding layer 504 and the thickness 514 of the thirdcladding layer 506. In an embodiment, the thickness 514 of the thirdcladding layer 506 may be less than the thickness 512 of the secondcladding layer 504. In some embodiments, the thickness 514 of the thirdcladding layer 506 may be greater than the thickness 512 of the secondcladding layer 504. In other embodiments, the thickness 514 of the thirdcladding layer 506 may be less than the thickness 512 of the secondcladding layer 504. In embodiments, the thickness 514 of the thirdcladding layer 506 may be between 1 μm and 4 μm. In some embodiments,the thickness 512 of the second cladding layer may be between 1 μm and 2μm.

It should be noted that thickness 510 of the first cladding layer 502,thickness 512 of the second cladding layer 504, and thickness 514 of thethird cladding layer 506 are shown for illustrative purposes only andmay not be to scale in accordance with embodiments of the disclosure.Furthermore, although waveguide 500 is illustrated as having threecladding layers, embodiments of the disclosure may include waveguideshaving any number of cladding layers, any number of refractive indexes,or any combination thereof.

FIG. 6 depicts a flow diagram of a method 600 for utilizing a gradedindex waveguide by a LIDAR system in accordance with implementations ofthe present disclosure. In embodiments, various portions of method 600may be performed by one or more components of LIDAR system 100 of FIG.1.

With reference to FIG. 6, method 600 illustrates example functions usedby various embodiments. Although specific function blocks (“blocks”) aredisclosed in method 600, such blocks are examples. That is, embodimentsare well suited to performing various other blocks or variations of theblocks recited in method 600. It is appreciated that the blocks inmethod 600 may be performed in an order different than presented, andthat not all of the blocks in method 600 may be performed.

At block 610, a waveguide receives an optical beam from an opticalsource of a LIDAR system, as previously described at FIG. 1. Thewaveguide may include a first cladding layer having a first refractiveindex and a second cladding layer having second refractive indexes thatinclude a range of different refractive indexes that are greater thanthe first refractive index. In embodiments, the waveguide may correspondto waveguide 300, as previously described at FIG. 3. In an alternativeembodiment, the optical source may provide the optical beam to awaveguide having three or more cladding layers, each having a differentrefractive index, as previously described at FIG. 5.

At block 620, the waveguide propagates the optical beam using the rangeof different refractive indexes of the waveguide to expand an opticalmode of the optical beam, as previously described.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present disclosure. Thus, the specific details set forth are merelyexemplary. While this specification contains many specificimplementation details, these should not be construed as limitations onthe scope of any inventions or of what may be claimed, but rather asdescriptions of features specific to particular embodiments ofparticular inventions. Certain features that are described in thisspecification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable sub-combination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a sub-combination or variation of a sub-combination.Moreover, the separation of various system components in the embodimentsdescribed above should not be understood as requiring such separation inall embodiments, and it should be understood that the described programcomponents and systems can generally be integrated together in a singlesoftware product or packaged into multiple software products. Particularembodiments may vary from these exemplary details and still becontemplated to be within the scope of the present disclosure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiments included inat least one embodiment. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.In addition, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or.”

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittent oralternating manner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Moreover, use of the term “an embodiment” or “one embodiment” or“an implementation” or “one implementation” throughout is not intendedto mean the same embodiment or implementation unless described as such.Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. asused herein are meant as labels to distinguish among different elementsand may not necessarily have an ordinal meaning according to theirnumerical designation.

What is claimed is:
 1. A light detection and ranging (LIDAR) apparatus,comprising: at least one waveguide comprising: a first cladding layercomprising a first refractive index; and a second cladding layercomprising a plurality of second refractive indexes to expand an opticalmode of an optical beam propagating within the waveguide, the pluralityof second refractive indexes comprising a gradient of refractiveindexes, and wherein the first refractive index is less than theplurality of second refractive indexes.
 2. The LIDAR apparatus of claim1, wherein the second cladding layer is comprised of indium phosphide(InP).
 3. The LIDAR apparatus of claim 1, wherein the gradient ofrefractive indexes increases linearly from a first surface of the secondcladding layer and a second surface of the second cladding layer.
 4. TheLIDAR apparatus of claim 1, wherein the gradient of refractive indexesincreases exponentially from a first surface of the second claddinglayer to a second surface of the second cladding layer.
 5. The LIDARapparatus of claim 1, further comprising: an optical detector to receivea combined signal comprising a target signal and a local oscillatorsignal associated with the optical beam.
 6. The LIDAR apparatus of claim1, the LIDAR apparatus is a frequency modulated continuous wave (FMCW)LIDAR apparatus.
 7. The LIDAR apparatus of claim 1, wherein the firstcladding layer has a first thickness, the second cladding layer has asecond thickness, and the first thickness is less than the secondthickness.
 8. A method comprising: receiving, by a waveguide, an opticalbeam from an optical source of a light detection and ranging (LIDAR)system, the waveguide comprising a first cladding layer comprising afirst refractive index and a second cladding layer comprising aplurality of second refractive indexes to expand an optical mode of anoptical beam propagating within the waveguide, the plurality of secondrefractive indexes comprising a gradient of refractive indexes, andwherein the first refractive index is less than the plurality of secondrefractive indexes; and propagating the optical beam using the gradientof refractive indexes of the waveguide to expand an optical mode of theoptical beam.
 9. The method of claim 8, wherein the gradient ofdifferent refractive indexes increases linearly from a first surface ofthe second cladding layer and a second surface of the second claddinglayer.
 10. The method of claim 8, wherein the second cladding layer iscomprised of indium phosphide (InP).
 11. The method of claim 8, whereinthe gradient of refractive indexes increases exponentially from a firstsurface of the second cladding layer to a second surface of the secondcladding layer.
 12. The method of claim 8, further comprising: splittinga portion of the optical beam to generate a local oscillator signal;receiving a target signal associated with the optical beam; combiningthe target signal with the local oscillator signal to generate acombined signal; and providing the combined signal to an opticaldetector.
 13. The method of claim 12, wherein the waveguide is includedin a frequency modulated continuous wave (FMCW) LIDAR system.
 14. Themethod of claim 8, wherein the first cladding layer has a firstthickness, the second cladding layer has a second thickness, and thefirst thickness is less than the second thickness.
 15. A light detectionand ranging (LIDAR) system, comprising: at least one waveguidecomprising: a first cladding layer comprising a first refractive index;and a second cladding layer comprising a plurality of second refractiveindexes to expand an optical mode of an optical beam propagating withinthe waveguide, the plurality of second refractive indexes comprising agradient of refractive indexes, and wherein the first refractive indexis less than the plurality of second refractive indexes.
 16. The LIDARsystem of claim 15, wherein the gradient of refractive indexes increaseslinearly from a first surface of the second cladding layer and a secondsurface of the second cladding layer.
 17. The LIDAR system of claim 15,wherein the gradient of refractive indexes increases exponentially froma first surface of the second cladding layer to a second surface of thesecond cladding layer.
 18. The LIDAR system of claim 15, furthercomprising: an optical detector to receive a combined signal comprisinga target signal and a local oscillator signal associated with theoptical beam.
 19. The LIDAR system of claim 15, wherein the LIDAR systemis a frequency modulated continuous wave (FMCW) LIDAR system.
 20. TheLIDAR system of claim 15, wherein the first cladding layer has a firstthickness, the second cladding layer has a second thickness, and thefirst thickness is less than the second thickness.